Iran’s interior ministry said President Mahmoud Ahmadinejad had moved into a strong lead with nearly 70 per cent of the votes tallied so far.
But his pro-reform rival, Hossein Mousavi, the former prime minister, said that he had won and warned of voter fraud.
The dispute flared up before the polls closed and heightened tensions across the capital, where emotions have run high in the streets. Mousavi indicated he might challenge the results.
The dramatic claims of victory capped a long day of voting that was extended for six hours to accommodate a huge turnout. During the voting, the government blocked text messages, a key campaigning tool for reformers, as well as some pro-Mousavi websites.
Security officials threatened to crack down on political gatherings or rallies before the final results were known.
Before the polling closed Mousavi declared himself “definitely the winner” based on “all indications from all over Iran.” He alleged widespread voting irregularities without giving specifics and hinted he was ready to challenge the final results.
Iran’s state news agency responded moments after Mr Mousavi spoke, and reported that Ahmadinejad was the victor. The report by the Islamic Republic News Agency gave no details.
With more than 10 million votes counted, Ahmadinejad had 68.8 percent and Mousavi had 28.8 percent, said Kamran Daneshjoo, a senior official at the Interior Ministry.